For cardiovascular health (not training, or endurance, or other reasons for exercising), how frequently is it recommended to do cardio? What is the minimum to have any effect, what is the maximum where not much is gained, and what is optimal?

Or to phrase it in a different way, is jogging for 15 minutes once a week any better than doing no exercise at all? Is running for three hours a day better than one hour a day? For a healthy lifestyle, how often should cardio be done? Again, I'm asking about exercising for cardiovascular health, not building stamina or training for a sport.

Intensity and duration depend on your goals. If you want endurance, jogging (or other aerobic exercises) for long distance/duration is what you're looking for. Basically, aerobic exercise (moderate heart rate) forces your body to adjust itself to maximize it's resources. If you want sheer power and/or ability to more quickly recover from hard short bursts then an anaerobic (which is still considered cardio) workout is what you're looking for. Ie. 'hard exercise' like sprinting, running stairs, etc but not lifting weights (that's a different class of exercise altogether).
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Evan PlaiceJul 15 '11 at 21:04

(cont) There are disadvantages to both types of workouts too. For instance, if you focus mostly on aerobic cardio, you'll find that it becomes increasingly difficult to increase intensity above your normal range. Conversely, while anaerobic cardio is also good for endurance it's not so good for long range endurance. While your cardiovascular system will become really good at recovering from high intensity exercises, you'll also increase muscle mass (which requires more energy). That means you'll run out of energy a lot faster and have to eat more often to avoid hitting an energy trough.
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Evan PlaiceJul 15 '11 at 21:12

4 Answers
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What it sounds like you are asking is how can you improve cardiovascular health most efficiently with the use of activity.

First off, heart health is mainly achieved through dietary adjustments but regular activity plays a vital role as well.

Any activity can improve the health of your heart provided you are reaching the right intensity. Indicators we use to determine level of intensity are breathing patterns, sweat production and heart rate. When these measurements reach a certain level for your age group and level of fitness, then you enter what some refer to as the "cardio zone". This simply means that the body's mechanisms for dealing with a stressful situation has begun to demand more out of your heart than what it is typically used to. This is the basis of muscle development. The heart (a muscle) must be challenged. Your goal here should be to maintain that intensity for about 5 minutes per session. You should have 3-6 sessions per week.

The great part about this method is that you can work as quickly or slowly as you want to reach the right intensity. And as you progress in your fitness, the rules don't change. 5 minutes of intensity will get the job done. But you will find it harder to reach the desired level of intensity as your heart becomes stronger.

As for minimums/maximums etc: The heart is perfectly capable of staying healthy and strong without ever reaching these levels of intensity. However, in our present society with so many poor choices of food out there, it is a good idea to supplement your health habits with good quality exercise. I think 5 minutes of elevated heart rate, sweat producing activity 3 times per week would be the minimum goal to shoot for.

When does it become overkill? That's a bit more subjective because the heart can and will continue to improve it's performance capability with increased demand. I'd say just do as much as you're comfortable with and provided you're reaching your intensity goals, be satisfied knowing that you are keeping your heart healthy and strong.

+1 good answer. The word 'fitness' itself is just a simplified term for VO2 Max; which is basically your body's max threshold for oxygen utilization during strenuous effort. It's counterproductive to try do any other sort of strenuous exercise without building cardio first. It's like going to war without first establishing a supply chain.
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Evan PlaiceJul 15 '11 at 20:59

This depends a great deal on the intensity of the exercise. According to Body By Science, a ~15 minute weekly session of extremely high-intensity weightlifting can serve all your cardio needs; conversely, you can spend all week doing something really low-impact and not see much benefit.

What we think of as "cardio" and "aerobics" is actually a body-wide process for metabolizing energy at the cellular level, and this is what particularly intense exercise targets.